A magnetic disk device is the mainstream of a file data storage device in an information processing system such as a personal computer, or the like, at present. The magnetic disk device has a lot of merits such as low price, large capacity, high reliability, high performance, etc. and is most popular. With the gradual increase of demands for reduction in size, improvement in portability, reduction in consumed electric power, etc., a semiconductor memory device using a semiconductor as a recording medium has begun to be highlighted because the semiconductor memory device is more excellent in portability and low consumed electric power than the magnetic disk device.
A file device using a nonvolatile memory as a recording medium is disclosed in JP-A-5-27924. This technique concerns a memory device using a flash memory as a recording medium, and particularly discloses a system in which the life of a flash memory limited in the number of times of erasing required for rewriting is prolonged. Specifically, the number of times by which erasing is allowed is averaged for all regions in a file storage device which is apt to be rewritten locally, so that the apparent life is prolonged.
In the aforementioned invention, the number of times of erasing is used as an indicator for expressing deterioration of each flash memory cell and considered on the basis of the guaranteed value of the erasable number of times specified as characteristic data of the flash memory. The guaranteed value of the erasable number of times specified for each of various flash memories is, however, an average value or a minimum value in terms of characteristic, while the number of times by which erasing is allowed in practice varies according to the individual memory cells. That is, all the cells do not necessarily become unusable even when the number of times by which erasing is allowed reaches the guaranteed value. Accordingly, while the number of times by which erasing is allowed may be counted as an indicator for deterioration of each cell, there is some case where it cannot be said that memory cells are used most efficiently.
Therefore, an object of the present invention is to provide a method or semiconductor memory device in which deterioration of a memory can be grasped without consciousness of the guaranteed number of times and the current number of times by which the aforementioned electrically erasable/rewritable nonvolatile memory is allowed to be erased/rewritten, and, further, to provide a memory management method or semiconductor memory device using the same.
Another object of the present invention is to provide a method or semiconductor memory device in which the aforementioned electrically erasable/rewritable nonvolatile memory can be used up to the detected actual erasable/rewritable limit.
A further object of the present invention is to provide a method or semiconductor memory device for detecting the number of times by which the aforementioned electrically erasable/rewritable nonvolatile memory is allowed to be actually erased/rewritten.